Prior art devices that attempt to optically detect human skin have typically suffered from a number of limitations. Among them have been the issues of temperature sensitivity, that is, performance fluctuations caused by variations in operating or environmental temperature; variations in output power of the source, or sensitivity of the sensor.
Firstly, the device as described may be sensitive to temperature fluctuations. Since the LED's used as optical sources have an optical output power that depends on the temperature of the LED, the precise output power of each of the LED sources may vary with ambient temperature and may vary due to self-heating or by heating from the control electronics. Additionally, since the sensitivity of the phototransistor used to detect the remitted light may depend on the temperature of the phototransistor, the measurement of the remitted light may vary due to self-heating or by heating from the control electronics. Sources that vary in output power will affect the amount of light remitted. A phototransistor which has a sensitivity to temperature will give a measurement that has some error in the actual amount of remitted light.
Further, skin sensors in the prior art typically indicate only a broad indication of the presence of skin, which is less than ideal in at least some instances. None of the prior art measures the actual spectral remission at multiple wavelengths and reports that spectral remission for each wavelength as the fractional amount of light remitted at each wavelength. Thus, in the present invention, the ratio of the amount of light remitted at each wavelength is divided by the amount of light of that wavelength incident on the skin, and this fraction is then available for further use.
In addition, the prior art is typically unable to ascertain the degree of pigmentation of the skin. This is desirable in at least some applications, especially for devices which rely upon thermolysis.
As a result, there are significant shortcomings with prior art devices, and there has been a long felt need for a device and a technique which overcomes or substantially reduces these limitations.